1. Field of the Invention
A great deal of interest is now focused on introducing and expressing foreign genes in mammalian cell lines. It is likely that many medically and commercially desirable proteins will have to be produced in mammalian cell lines because of post-translational modification, such as glycosylation, which is difficult to mimic in prokaryotic and lower eukaryotic hosts. In addition, much research on mammalian cells must be performed on mammalian hosts. For these reasons, it is desirable to provide regulatory systems and vectors which are capable of replication and expression in mammalian hosts.
2. Description of the Prior Art
A subgenomic fragment of the bovine papilloma virus (BPV) induces malignant transformation in certain target cells, as reported by: Lowy, et al. (1980) Nature 287:72-74. The BPV subfragment exists as a self-replicating episome in certain hosts as reported by: Law, et al. (1981) Proc. Nat. Acad. Sci USA 78:2722-2731. Chimeric BPV vectors containing various genes can exist in cells as episomes. See, Sarver, et al. (1981) Mol. Cell Biol. 1:486-496 where the cells containing the vector were selected on the basis of their transformed phenotype.